Ok, it's not hard, but it is quite some work. Backup your stuff, download the resourcehacker, open the sylenth1 DLL, right click on the 'PNG' tree and save all PNG resources. This will extract all graphical resources from Sylenth1. Eventually you will have to replace all of these with your new ones.
You can open the PNG's you saved with a graphics editor like the mspaint that's standard in your windows installation.
Now, check that first one (the 710 kb file), it's the main layout. You'll see black holes in it, where the buttons should come. Get yourself one of the skins you like, and make black holes on the exact same places. Save it (as a PNG). Then, in reshacker, expand that first PNG (10001), click delete resource. Then select 'action' in the menu, 'add a new resource', select the PNG you made with paint, enter 'PNG' as resource type, '10001' as resource name (because you deleted that one) and 1033 as language. Save it and load up your vst hosts to see if it worked. It should have.
However, as you'll see, only the main skin was replaced. You'll have to repeat the above for all resources (buttons etc), so that's why it's quite a bit of work, especially the buttons (see resource 2 for example, you'll get the idea why it's quite some work)